Too many men and woman have died in the effort to explore space—and too many of them are unknown. NASA has set aside a day to honor them all. What follows is a remembrance the Russians and Americans who lost their lives during missions, and the NASA astronauts who lost theirs during training.

Soyuz 11: Georgi Dobrovolski, Victor Patsayev, Vladislav Volkov

Russian cosmonauts Georgi Dobrovolski (1928 - 1971), Vladislav Volkov (1935 - 1971) and Viktor Patsayev (1933 - 1971) of the Soyuz 11 space mission, June 1971. All three men died during their return to Earth, following a successful docking with the Salyut 1 space station.

The most unsettling parts of any space flight are those that occur when the crew is out of contact with the ground. In the earlier days of the space program, orbiting astronauts would move in and out of blackouts as they orbited the Earth, flying in range of tracking stations and then out again. Global communications satellites and listening posts changed that, but blackouts still occurred when lunar astronauts were on the far side of the moon and, on every mission up to the present, during reentry, when a storm of ionized gasses around the superheated spacecraft block radio signals. Crews that enter ionization blackout alive and well have always emerged from it the same way—except once. The Soyuz 11 cosmonauts had docked with the Salyut 1 space station, but had trouble latching firmly to it and were never able to enter. Still, they spent more than three weeks in space and made what seemed a nominal reentry. What no one could know was that explosive bolts designed to separate the reentry module from the rest of the ship misfired, causing a severe jolt that in turn caused a pressure valve beneath one of the seats to open. The cabin rapidly lost all its atmosphere before descending into thicker air and slowly repressurizing. But it was too late for the crew. Only silence came from the ship when it emerged from blackout and when the recovery team reached the spacecraft and opened it up, they found all three cosmonauts dead.